The Fiber Bragg grating (FBG) is a well established technology for applications in optical telecommunications, especially for Wavelength-Division-Multiplexing (WDM). Basically, a FBG reflects light propagating into an optical fiber at a wavelength known as the Bragg wavelength, which is determined by the period of the grating and the fiber effective index. A chirped FBG, in which the grating period varies as a function of the position along the fiber, is a well known solution for compensating the chromatic dispersion of an optical fiber link (see for example F. Ouellette, “Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides,” Opt. Lett., Vol. 12, pp. 847-849 (1987); and R. Kashyap, “Fiber Bragg gratings,” Academic Press, 458p. (1999)). Such a grating can compensate for the dispersion accumulated over an optical fiber link by providing a group delay that varies as a function of wavelength in a manner opposite to that of the group delay in the fiber link.
From the many available methods for the photo-inscription of FBG, the use of a phase mask is recognized as the best choice for obtaining good optical performance (see for example U.S. Pat. No. 5,367,588 (HILL et al) and U.S. Pat. No. 5,327,515 (ANDERSON et al). The phase mask acts as a master that is used to replicate FBGs with the same optical characteristics on pieces of optical fiber in a fast and repeatable way, allowing for efficient mass production. The phase mask can contain all the information about the FBG to be written or only part of it, depending on the desired balance between ease of fabrication and flexibility.
Although the use of a phase mask was initially limited to the inscription of the period profile of a single channel FBG, recent advances have made possible the encoding of the multi-channel character through phase sampling, as shown in U.S. Pat. No. 6,707,967 (ROTHENBERG et al). In a further development, the in-mask encoding of the apodization profile of the FBG was proposed in U.S. patent application published under no. 2004/0264858 (ROTHENBERG). The whole information related to a FBG can thus be encoded into the phase mask, hence maximizing its manufacturability. In practice, this means that a binary phase mask with properly positioned groove edges can be used to write a FBG with a complex spectral response using a uniform exposition to actinic radiation.
While FBGs were initially considered as narrow-band single-channel devices, several advances have been made during the past few years leading to the fabrication of multi-channel FBGs. Assignee's own U.S. Pat. No. 6,865,319 (PAINCHAUD) teaches that the multi-channel optical response can be obtained by superposing different FBGs on the same piece of fiber, each of them being associated with a specific WDM channel. This has the advantage of maximizing flexibility since the optical response of the final structure can be tailored on a per-channel basis. However, a long writing time is required for the channel per channel inscription and the required total index change is high and increases with the number of superposed FBGs. For example, FIG. 1A (PRIOR ART) shows the simulated amplitude profile of the change in index of refraction resulting from the superposition of 8 grating components providing a dispersion varying from 400 to 1800 ps/nm in channels separated by 100 GHz, as illustrated in FIGS. 1E and 1F (PRIOR ART). FIG. 1B (PRIOR ART) shows the corresponding period profile. FIG. 1D shows that reflectivity peaks are created only in the spectral region of interest, i.e. there is no side band produced by this process. FIG. 1G (PRIOR ART) shows the group delay ripple, defined as the deviation of the group delay spectrum from a straight line. FIG. 1C shows the Fourier spectrum of the phase profile. In this example, each FBG component is considered to be written using an apodization technique such as the moving phase mask method described in U.S. Pat. No. 6,072,926 (COLE et al). If the maximum index change is desired to be minimized, the relative phases between the 8 components can be selected in an appropriate manner such as using Barker series, as taught in L. Bömer, M. Antweiler, “Polyphase Barker sequences,” Electron. Lett., Vol. 25 (23), pp. 1577-1579 (1989). In the example of FIG. 1, the 8 components are centered on the overall structure, although there are of different lengths. Other spatial management could also be of interest for minimizing the maximum index change. For example some components could be located at one edge of the structure while other at the center or at the other edge.
The prior art embodiment of grating superposition at the writing stage is somewhat inefficient as a uniform index increase is created during the inscription of each individual component. These uniform index increases add up linearly as the grating components are superposed, whereas partial fringe wash-out during the superposition process reduces the overall index modulation. This uniform index offset shifts uniformly the spectral response of the overall FBG but does not contribute otherwise in shaping the grating optical response. The ratio of the uniform index offset on the peak index change increases with the number of grating components. Accordingly, the inefficiency of superposing individual gratings is worst for high-channel-count structures. An ideal writing procedure would produce the same index modulation but around an average index change lowered by the index offset.
FBG sampling, as for example described in U.S. Pat. No. 6,707,967 (ROTHENBERG et al), is an attractive alternative to superposing FBGs, especially when performed on the grating phase rather than on the grating amplitude. In this technique, the multi-channel character is encoded directly into the phase mask and the whole complex FBG structure can be created in a single inscription step. This method has the advantage of being fast and suitable for mass production. However, the achievable optical characteristics are somewhat limited. Uniform sampling produces identical replicas in the spectral response (J. E. Rothenberg, R. F. Caldwell, H. Li, Y. Li, J. Popelek, Y. Sheng, Y. Wang, R. B. Wilcox and J. Zweiback, “High-channel count fiber Bragg gratings fabricated by phase-only sampling,” Proc. of OFC 02, pp. 575-577 (2002)). Chirped sampling allows dispersion to differ somewhat from channel to channel, but in a limited manner (M. Morin, M. Poulin, A. Mailloux, F. Trépanier and Y. Painchaud, “Full C-band slope-matched dispersion compensation based on a phase sampled Bragg grating,” Proc. of OFC 04, paper WK1 (2004)). The dispersion variation comes along with a concomitant bandwidth variation from channel to channel, clearly an undesirable feature. The channel-to-channel dispersion variation that can be achieved with chirped sampling is thus rather limited.
Lee et al. (“Bandwidth equalization of purely phase-sampled fiber Bragg gratings for broadband dispersion and dispersion slope compensation,” Opt. Express, Vol. 12 (23) p. 5595-5602 (2004)) proposed an approach for fabricating a multi-channel dispersion compensation FBG in which the dispersion varies but the bandwidth remains relatively uniform from channel to channel. This approach is based on phase sampling in which the coupling coefficient is also chirped (in addition to the grating and sampling periods being chirped). However, this approach only provides some improvement for still modest channel-to-channel dispersion variations.
There is thus a need for a multi-channel dispersion compensating device in which the dispersion can significantly differ from channel to channel whilst the channel bandwidth remains relatively uniform. Such a device would be of particular interest to compensate for the chromatic dispersion accumulated over many WDM channels along transport fibers such as NZ-DSF fibers, in which the spectral variation of the dispersion is relatively large, or to compensate for the residual dispersion after propagation over a long link with incomplete dispersion slope compensation. There is also a need that such a device be produced in a manner compatible with mass production. Use of a complex phase mask containing most of the FBG structure complexity (or at least the multi-channel character) is thus certainly of interest.